Epoxy resins are widely used for electronic parts, electrical devices, automobile parts, FRP, sporting goods, and the like, due to their excellent adhesion, heat resistance, and molding properties. In particular, it is required that copper-clad laminates and sealants used in electronic parts and electrical devices must be flame retardant so as to assure safety, i.e., prevention of fire and delay of combustion. Brominated epoxy resins and the like having these properties have been used. The brominated epoxy resins are considered to be useful electronic and electrical materials because flame retardancy is imparted by introduction of a halogen, especially bromine and the epoxy group exhibits high reactivity and imparts an excellent curing property.
However, there is a recent tendency that electrical devices are developed such that the size and weight of the devices are reduced and circuits therein are miniaturized. Under the situation where these demands must be satisfied, halides having a high specific gravity are not preferable from the point of reduction in weight. In addition, after the use thereof at a high temperature for a long period of time, separation of halides may occur. It may lead to corrosion of fine wirings. Further, as harmful compounds such as halides and the like are produced when used electronic parts and electrical devices are burned, use of halogens is being considered as a problem from the view point of assuring the environment safety.
Recently, a large number of inorganic materials such as aluminum hydroxide, phosphorus compounds, nitrogen compounds, and the like as substituting materials have been studied. In particular, among them, flame retardant formulations using a phosphorus compound have been studied recent years. Addition of phosphates, red phosphorus, and the like as phosphorus sources which impart flame retardancy to epoxy resins is disclosed. Phosphates affect the anti-migration property because phosphates are hydrolyzed to release acids. Although red phosphorus exhibits high flame retardancy, it is designated as a hazardous substance by the Fire Defense Law and a trace amount of a phosphine gas is produced under atmosphere at high temperature and high humidity. Therefore, it has been studied to use a compound represented by Formula 3 and disclosed in Non-patent literature publications 1 and 2 and Patent publication 1 so as impart flame retardancy to epoxy resins.

Regarding imparting flame retardancy to resins using the compound represented by Formula 3, a phosphorus-containing epoxy resin obtained by reacting the compound with an epoxy resin described in Patent publication 2 and an example of applying the compound to a cyanate resin described in Patent publication 3 obtained by the reaction with a cyanate ester compound were known. It is disclosed that flame retardancy is imparted by modifying the resin with phosphorus in the methods. As described above, the compound represented by Formula 3 is extremely important in the means for imparting flame retardancy without using a halogen.
A method for producing the compound represented by Formula 3 is specifically described in, for example, Non-patent literature publication 2 and Patent publication 1. However, a recrystallization step is necessary to increase the purity of the compound in the method. The yield of the compound is low by the method. Therefore, the compound could not be prepared with high purity and in sufficient yield by any conventional methods. In addition, as equipments must be used for a long period of time during the recrystallization step, productivity is low.
The object to increase the productivity of the compound represented by Formula 3 is disclosed in Patent publication 4. The object is achieved by a method for producing a flame retardant epoxy resin which comprises reacting an epoxy resin with a compound represented by Formula 2 and a compound represented by Formula 1 in the presence of an organic solvent. It is described that after the compound represented by Formula 3 is obtained by reacting the compound represented by Formula 2 with the compound represented by Formula 1, the epoxy resin is further reacted with the above reaction product to obtain a phosphorus-containing epoxy resin. As the compound represented by Formula 2 and the compound represented by Formula 1 per se charged into the system can be introduced into the resin, the yield of the compound represented by Formula 3 is increased. However, there are problems that the resin appearance, curing reactivity, heat resistance, anti-migration property, and the like are influenced by side-products produced by the reactions.
The inventors of the present application keenly studied phosphorus-containing epoxy resins and found that the physical properties of a phosphorus-containing epoxy resin obtained by reacting the phosphorus-containing phenol compound represented by Formula 3 with an epoxy resin at the predetermined molar ratio are significantly influenced by the quality of the phosphorus-containing phenol compound and filed Patent Application No. 2008-023014 and Patent Application No. 2008-023015. Namely, due to the presence of a trace amount of a reaction side-product, such as a phosphorus-containing mono-phenol compound, as an impurity contained in the compound represented by Formula 3, the curing time of the phosphorus-containing epoxy resin is remarkably prolonged and adhesion property thereof becomes poor. As the curing reactivity is changed by the presence of an impurity component, it was predicted that problems of reduction in adhesion property and heat resistance of the resins, due to the insufficient curing level and declined productivity, due to curing time delay would occur. It was also predicted that problems of reduction in adhesion force and insulation failure would occur in the advanced electronic material field in which electronic circuits are being miniaturized, depending on the curing reaction conditions. Therefore, it is clear that the purity of the compound represented by Formula 3 which is a material for phosphorus-containing epoxy resins is very important.
Considering the above points, when flame retardancy is imparted to epoxy resins, it is necessary to use the high-purity phosphorus-containing phenol compound represented by Formula (3). However, a conventional method for obtaining the phosphorus-containing phenol compound with high purity and in high yield was not known.
Other than modification of epoxy resins with phosphorus, a method for simply adding phosphorus to a curable epoxy resin composition was disclosed as a method for imparting flame retardancy using the compound represented by Formula 3. Patent publications 5 to 8 describe the method. The publications also describe that curable resin compositions obtained by adding the predetermined amount of the compound represented by Formula 3 to an epoxy resin, a curing agent, and a curing accelerator and cured products thereof exhibit flame retardancy. Patent publication 8 describes that flame retardancy and heat resistance are increased by adding the compound to an epoxy resin. However, as the compound represented by Formula 3 exhibits poor solvent solubility and crystals thereof are easily formed, the compound can be added only in an amount such that crystals thereof are not formed in the technical filed where miniaturization and uniform curing are required, such as the electronic material field. Therefore, it was not considered that the compound can be a main component which imparts flame retardancy. There was a problem that the compound can be only used as an auxiliary compound for imparting flame retardancy.